With the popularization of small electronics (for example, mobile telephone, PDA, laptop computer, DVD, liquid-crystal TV, digital still camera, mobile music machinery, etc.), there is an increasing demand for small-sized electro-acoustic transducers, such as small-sized speakers (generally called microspeakers), small-sized receivers and further microphones, earphones and others used in those electronics.
In general, a diaphragm for speakers is required to have a low density so as to maintain the acoustic radiation sound pressure level, to have high rigidity for inhibiting strain so as to increase the allowable input resistance, and additionally to have a large specific modulus of elasticity so as to broaden the reproduction frequency band region, and to have a large internal loss for suppressing the divided vibration of the diaphragm so as to flatten the frequency characteristics. In use near a speaker driving source, voice coil, or in use in in-vehicle speakers, the diaphragm may be exposed to high temperature for a long period of time, and therefore it must have heat resistance fully bearable to such use conditions.
On the other hand, with the recent background of mobile or ubiquitous society or music source digitalization, various small electronics are made to have higher function and higher performance. Even in the speakers to be used in these, for example, in the speakers in mobile telephones, the input-output resistance level required for the diaphragm is being improved to be at least from 0.5 to 0.6 W or so (at present, the uppermost limit is 1.2 W or so) for high-output models, as compared with popular models with 0.3 W or so. At present, there are many types of models with from 0.6 to 0.8 W or so, and the ratio of models with more than 1.0 W is low.
Regarding diaphragms for speakers, for example, Patent References JP-A-60-139099, JP-A-59-63897, and JP-A-2002-291092 disclose a diaphragm for speakers, formed of a film of an aromatic polysulfone resin, concretely a polyether sulfone resin. These patent references disclose that when a film of a polyether sulfone resin is used, the formability, the heat resistance and the acoustic characteristics of the diaphragm for speakers are excellent. However, the diaphragm for speakers described in these patent references is problematic in that its durability in high-output operation is insufficient and the diaphragm may be often cracked or broken. In these patent references, nothing is described or investigated relating to the structure of an aromatic polysulfone resin, in particular to a diaphragm for speakers formed of a film of an aromatic polysulfone resin having a specific repetitive unit and the durability thereof in high-output operation.
Japanese Patent JP-A-58-222699 discloses a diaphragm for speakers, formed of an aromatic polyether ketone resin, concretely a polyether-ether ketone resin. This patent reference says that the modulus of elasticity of the film of a polyether-ether ketone resin is 30,000 kg/cm2 (about 3000 MPa) and is high, and therefore the film is excellent in high-pitched tone reproduction. However, this patent reference has no description of microspeakers and low-pitched tone reproduction with microspeakers, or has neither description nor investigation of the durability in high-output operation of a diaphragm formed of a mixture of an aromatic polyether ketone resin and any other resin and of the formability of the diaphragm.
On the other hand, as a diaphragm for speakers excellent in the durability in high-output operation, used is a biaxially-stretched thermally-fixed film of a polyethylene naphthalate resin (PEN) film or a polyphenylene sulfide resin (PPS) film. However, these films are crystalline and their rigidity is too high, and therefore, their minimum resonance frequency (f0: f zero) is high and their acoustic characteristics are insufficient, for example, their low-pitched tone reproducibility is poor, and in particular, are problematic in that their formability into diaphragms (by press forming or vacuum forming) and their forming cycle are inferior to amorphous resins (polyetherimide, etc.) having a high glass transition temperature (Tg).